This invention relates to the configuration and manufacture of light-emitting devices suitable for use in flat-panel displays such as flat-panel cathode-ray tube (xe2x80x9cCRTxe2x80x9d) displays.
A flat-panel CRT display is formed with an electron-emitting device and an oppositely situated light-emitting device. The electron-emitting device, or cathode, contains electron-emissive elements that emit electrons across a relatively wide area. An anode in the light-emitting device attracts the electrons toward light-emissive regions distributed across a corresponding area in the light-emitting device. The anode can be located above or below the light-emissive regions. In either case, the light-emissive regions emit light upon being struck by the electrons to produce an image on the display""s viewing surface.
FIG. 1 presents a side cross section of part of a conventional flat-panel CRT display such as that described in U.S. Pat. No. 5,859,502 or U.S. Pat. No. 6,049,165. The flat-panel CRT display of FIG. 1 is formed with electron-emitting device 20 and light-emitting device 22. Electron-emitting device 20 contains backplate 24 and overlying electron-emissive regions 26. Electrons emitted by regions 26 travel toward light-emitting device 22 under control of electron-focusing system 28. Item 30 represents an electron trajectory.
Light-emitting device 22, a partial plan view of which is shown in FIG. 2, contains faceplate 32 coupled to electron-emitting device 20 through an outer wall (not shown) to form a sealed enclosure maintained at a high vacuum. Light-emissive regions 34 overlie faceplate 32 respectively opposite electron-emissive regions 26. When electrons emitted by regions 26 strike light-emissive regions 34, the light emitted by regions 34 produces the display""s image on the exterior surface (lower surface in FIG. 1) of light-emitting device 22. Contrast-enhancing black matrix 36 laterally surrounds regions 34.
Light-emitting device 22 also contains light-reflective layer 38 situated over light-emissive regions 34 and black matrix 36. Regions 34 emit light in all directions. Part of the light thus travels backward toward the interior of the display. Layer 38 reflects some of the rear-directed light forward to increase the intensity of the image.
Light-reflective layer 38 typically consists of aluminum, a silvery white metal which is highly reflective of visible light and a good conductor of electricity. Layer 38 is commonly exposed to air at some point during the display fabrication process. Inasmuch as aluminum is of relatively high chemical reactivity, a native coating (not shown) of aluminum oxide normally forms along the outside surface of layer 38 during the exposure to air. The native aluminum oxide coating is quite thin, typically 1-5 nm in thickness.
Light-reflective layer 38 functions as the display""s anode. For this purpose, layer 38 receives a high electrical potential that attracts electrons toward light-emitting device 22. Because layer 38 is located above light-emissive regions 34, electrons emitted by regions 26 pass through layer 38 and the overlying native oxide coating before striking light-emissive regions 34. By having layer 38 located above regions 34, the display of FIGS. 1 and 2 avoids the loss in image intensity that occurs in a flat-panel CRT display where light emitted by the light-emitting device must pass through the anode, typically transparent but still partially light-absorbent, before reaching the viewing surface.
A disadvantage of the conventional display of FIGS. 1 and 2 is that the electrons emitted by regions 26 lose some energy when they pass through light-reflective layer 38 and the overlying native oxide coating. Also, instead of passing through layer 38 and the oxide coating, some of the electrons emitted by regions 26 (a) scatter backward off layer 38 or/and the oxide coating or (b) cause layer 38 or/and the oxide coating to emit secondary electrons. Some of the backscattered and secondary electrons strike the interior of the display at such locations as to cause the image to be degraded. In addition, the native oxide coating along light-reflective layer 38 forms part of the interior surface of the display of FIGS. 1 and 2. Contaminants, such as oxygen and other chemically reactive gaseous species, commonly adhere to the oxide coating. As electrons (both primary and secondary) strike the oxide coating, these contaminants can be released into the display""s interior and cause damage.
Washington, xe2x80x9cColor Display Using the Channel Multiplier CRTxe2x80x9d, Procs. SID, 1998, pages 23-31, discloses a flat channel multiplier CRT display in which a carbon coating is applied to a light-reflective aluminum layer situated over the interior surface of the display""s fluorescent screen. Electrons pass through the carbon coating before passing through the aluminum layer to strike the screen. Washington reports that the carbon coating reduces both the number and energy of backscattered electrons. Although Washington is of interest, Washington presents a narrow solution to the electron backscattering problem and does not deal generally with electron backscattering, secondary electron mission, and display contamination problems that occur as electrons impinge on a light-reflective layer such as layer 38 in the conventional display of FIGS. 1 and 2.
It is desirable to reduce the loss in electron energy that occurs when electrons pass through a light-reflective layer in a flat-panel CRT display before striking light-emissive regions in the display""s light-emitting device. It is also desirable to have a general methodology for reducing electron backscattering and secondary electron emission that occur as electrons emitted by the display""s electron-emitting device impinge on the light-reflective layer. Furthermore, it is desirable to reduce the amount of contaminants released into the interior of the display as electrons impinge on the light-reflective layer.
The present invention furnishes a light-emitting device containing a plate, a light-emissive region overlying the plate where the plate is generally transmissive of visible light, and a light-reflective layer extending over the light-emissive region. The light-emitting device of the invention is suitable for use in a flat-panel display, especially a flat-panel CRT display in which an electron-emitting device is situated opposite the light-emitting device. The electron-emitting device emits electrons which pass through the light-reflective layer and strike the light-emissive region, causing it to emit light.
Compared to a conventional light-emitting device having an aluminum light-reflective layer covered with a native coating of aluminum oxide and situated in generally the same relative location as the light-reflective layer in the light-emitting device of the invention, the present light-emitting device is configured to achieve one or more of the following characteristics: (1) reduced electron energy loss as electrons pass through the light-reflective layer, (2) gettering in the immediate vicinity of the light-reflective layer for reducing the amount of damage caused by contaminants, especially contaminants released close to the light-reflective layer, (3) reduced electron backscattering as electrons impinge on the light-reflective layer from above the light-emitting device, (4) reduced secondary electron emission as electrons impinge on the light-reflective layer from above the light-emitting device, and (5) reduced chemical reactivity along the light-reflective layer.
In a first aspect of the invention, the light-reflective layer contains non-aluminum metal consisting of at least one of lithium, beryllium, boron, sodium, and magnesium. The energy lost by an electron in passing through a layer depends on the number of protons that the electron effectively encounters (interacts with) during its passage through the layer. In turn, the number of protons encountered by an electron passing through a layer depends on the layer""s thickness, the angle at which the electron impinges on the layer, and the average volumetric density of protons in the layer. Each of lithium, beryllium, boron, sodium, and magnesium is of lower average volumetric proton density than aluminum. As a result, electrons lose, on the average, less energy in passing through the present light-reflective layer than through an equally thick aluminum layer.
The light-reflective layer in the present light-emitting device typically includes aluminum in addition to one or more of lithium, beryllium, boron, sodium, and magnesium. Because the present light-reflective layer contains one or more of these five non-aluminum metals, the electron energy lost in passing through the light-reflective layer is, on the average, again less than that lost in passing through an equally thick pure aluminum layer. By reducing the electron energy loss through the light-reflective layer, the present light-emitting device operates more efficiently than an otherwise equivalent prior art light-emitting device.
In a second aspect of the invention, the light-reflective layer is implemented as a getter for sorbing (adsorbing or absorbing) contaminant gases, especially sulfur a common constituent of the light-emissive region. The light-reflective getter layer contains one or more of magnesium, chromium, manganese, cobalt, copper, molybdenum, palladium, silver, platinum, and lead, each of which is suitable for sorbing sulfur. When sulfur is present in the light-emissive region, electrons striking the light-emissive region may cause it to outgas sulfur in the form of atomic/molecular sulfur or/and gaseous sulfur-containing compounds. Inasmuch as the light-reflective getter layer is very close to the light-emissive region, the light-reflective getter layer can sorb outgassed sulfur before it leaves the immediate vicinity of the light-reflective layer and causes damage elsewhere. The same applies to other contaminants which are released in the immediate vicinity of the light-reflective getter layer and which are readily sorbable by its getter material.
In a third aspect of the invention, an overcoat layer overlies the light-reflective layer above the light-emissive region. Relative to an imaginary native aluminum oxide coating formed along an imaginary aluminum layer and subjected to electrons which impinge on the native oxide coating at generally the same energies and impingement angles as electrons impinge on the overcoat layer, the overcoat layer provides at least one of (a) lower chemical reactivity than the native oxide coating, (b) lower secondary electron emission per unit area than the oxide coating, and (c) lower electron backscattering per unit area than the oxide coating.
The term xe2x80x9cimaginaryxe2x80x9d is used here in describing the native aluminum oxide coating because the oxide coating, although serving in conjunction with the imaginary aluminum layer as a reference or baseline for comparisons of chemical reactivity, secondary electron emission, and electron backscattering, is not actually present as a surface layer in the active portion of the electron-emitting device of the invention. Instead of being described as imaginary, the oxide coating could be described as a reference or baseline. Similar comments apply to the use of xe2x80x9cimaginaryxe2x80x9d in describing the aluminum layer covered by the oxide coating.
Materials especially attractive for the overcoat layer include beryllium, boron, chromium, silver, gold, beryllium oxide, boron nitride, boron oxide, aluminum nitride, silicon nitride, silicon oxide, vanadium oxide, chromium oxide, cerium oxide, and neodymium oxide depending on which of the preceding chemical reactivity, secondary electron emission, and electron backscattering properties is, or are, to be provided by the overcoat layer. Magnesium, silicon, germanium, tin, lead, boron-magnesium, vanadium phosphorus oxide, silver oxide, and europium oxide are all attractive for the overcoat layer. Other materials suitable for the overcoat layer are cobalt, ruthenium, neodymium, iridium, platinum, lithium-aluminum, beryllium-boron, beryllium carbide, beryllium-aluminum, boron-aluminum, sodium carbide, sodium nitride, sodium oxide, sodium-aluminum, magnesium-aluminum, copper oxide, and molybdenum oxide.
The overcoat layer may be implemented as two or more layers, each providing at least one of the preceding chemical reactivity, secondary electron emission, and electron backscattering properties. When reduced chemical reactivity is furnished by the overcoat layer in a multi-layer implementation, the uppermost of the layers provides the reduced chemical reactivity. All of the materials identified in the previous paragraph are variously suitable for the different layers in multi-layer implementations of the overcoat layer. In addition, carbon is especially attractive for use in multi-layer implementations. Other attractive or suitable candidates for use in multi-layer implementations are iron, nickel, niobium, molybdenum, and barium.
Reducing chemical reactivity along the light-reflective layer according to the teachings of the invention leads to a reduction in device contamination that occurs as electrons impinge on the light-reflective layer. By reducing electron backscattering or/and secondary electron emission according to the invention""s teachings, image degradation caused directly by such electron backscattering or/and secondary electron emission is reduced. Also, display contamination that results from outgassing caused by electron backscattering or/and secondary electron emission is reduced. The result is an improvement in device performance or/and lifetime.
A getter layer, referred to as the overcoating getter layer, overlies the light-reflective layer in a fourth aspect of the invention, the overcoating getter layer can lie fully above the light-emissive region. Alternatively or additionally, the overcoating getter layer can lie above a light-blocking region situated to the side of the light-emissive region below the light-reflective layer. In all of these cases, the overcoating getter layer is close to the light-emissive region.
Candidate materials for the overcoating getter layer are the metals magnesium, chromium, cobalt, copper, palladium, silver, platinum, and lead, along with oxides of magnesium, chromium, manganese, cobalt, nickel, and lead. All of these materials can readily sorb sulfur. Should sulfur or similar contaminants outgas from the light-emissive region and pass through the light-reflective layer, the overcoating getter layer can sorb these contaminants. Importantly, the close proximity of the overcoating getter layer to the light-emissive region enables the getter layer to sorb these contaminants before they leave the immediate vicinity of the light-emissive region and cause damage elsewhere. The overcoating getter layer is also well suited for sorbing contaminants which are released by the light-blocking region and pass through the light-reflective layer.
In a fifth aspect of the invention, a transparent undercoat layer is situated between the light-emissive region and the light-reflective layer. Compared to an imaginary native coating of aluminum oxide formed along an imaginary aluminum layer, the undercoat layer more strongly inhibits the light-reflective layer from undergoing chemical reactions along its lower surface, i.e., along where the light-reflective layer is closest to the undercoat layer, than does the imaginary native coating inhibit the imaginary aluminum layer from undergoing chemical reactions along the interface between the native coating and the aluminum layer. Due to presence of the undercoat layer, damaging chemical compounds such as opaque materials which could degrade the efficiency of the light-reflective layer are less likely to form along its lower surface.
Especially attractive candidates for the undercoat layer are silicon nitride, aluminum nitride, and chromium oxide. Other suitable candidates for the undercoat layer are the metal oxides silicon oxide, magnesium oxide, zirconium oxide, indium oxide, indium tin oxide, and tin oxide. Forming the undercoat layer with one or more of those metal oxides, including chromium oxide, stabilizes the lower surface of the light-reflective layer against later exposure to oxygen, a highly reactive gas to which the light-reflective layer is typically exposed subsequent to its formation. A similar benefit can be achieved by forming the undercoat layer with aluminum oxide of greater thickness than the imaginary native aluminum oxide coating.
A getter layer, referred to as the undercoating getter layer, lies under the light-reflective layer in a sixth aspect of the invention. In particular, the undercoating getter layer lies between the light-reflective layer and a light-blocking region provided to the side of the light-emissive region below the light-reflective layer. As such, the undercoating getter layer is quite close to the light-emissive region.
Suitable materials for the undercoating getter layer include the metals magnesium, chromium, manganese, cobalt, nickel, copper, palladium, silver, platinum, and lead, along with oxides of magnesium, chromium, manganese, cobalt, nickel, and lead. With nickel being particularly suitable for sorbing sulfur, all of these materials are particularly suitable for sorbing sulfur. The undercoating getter layer can thereby readily sorb sulfur. Since the undercoating getter layer is quite close to the light-emissive region, the sorbing of sulfur and other similar contaminants can be done before these contaminants escape the immediate vicinity of the light-emissive region and cause damage elsewhere. The undercoating getter layer is also well located for sorbing contaminants released by the light-blocking region.
Fabrication of a light-emitting device in accordance with the invention entails providing a light-emissive region over a plate. When the present undercoat layer is to be included in the device, the undercoat layer is formed over the light-emissive region. When the undercoating getter layer is to be included in the device, the undercoating getter layer is formed over a light-blocking region provided to the side of the light-emissive region. The light-reflective layer is then formed over the undercoat layer or the undercoating getter layer. If neither the undercoat layer nor the undercoating getter layer is to be present, the light-reflective layer is simply formed over the light-emissive region. When the overcoat layer or the overcoating getter layer is to be included in the device, the overcoat layer or the overcoating getter layer is formed over the light-reflective layer.
In short, the light-emitting device of the invention is configured to improve image clarity or/and increase device lifetime without significant loss in image intensity. The present light-emitting device can readily be manufactured in a large-scale production environment. Accordingly, the invention provides a substantial advance over the prior art.